The so-called keyed insertion technique, by which a part of one television picture is inserted into another picture to produce a special effect, is frequently used in television broadcasting. One example of such a technique is chroma-key insertion wherein a part of a first picture is designated by a chroma-key signal produced from that picture and the designated part is inserted into a second picture. However, since a chroma-key signal undergoes a change in position and dimension with the movement of a television camera, employed for the pickup of the second picture, the shooting angle of the camera must be modified accordingly. This creates serious difficulties and to eliminate the difficulties, a method has been proposed in which the picture to be inserted is compressed in accordance with the chroma-key signal. (See for example Japanese Patent Publication No. 53-9896.) With this technique, the position and dimensions of the picture are determined by comparing the chroma-key signal with a standard television frame.
Referring to FIGS. 1A-1E, it is assumed that the picture of FIG. 1B is compressed into the size of a chroma-key signal shown in FIG. 1C to produce a picture (FIG. 1D) and that the picture of FIG. 1D is inserted into the picture of FIG. 1A to produce the picture of FIG. 1E. In this example, the chroma-key signal of FIG. 1C serves as the standard signal to designate into to what position and what dimensions the picture of FIG. 1D should be compressed. Generally, the chroma-key signal of FIG. 1C is produced by mixing in appropriate proportions of the blue component, as a main constituent, out of the various chromatic components (the red, green and blue components) constituting the picture of FIG. 1A, with the two other components. It is therefore customary, when a chroma-key signal is to be produced, to compose the picture of FIG. 1A by leaving blue that part of the picture which is crosshatched.
In the temporal relationship between a chroma-key signal and the compressed picture corresponding to it, the latter tends to include a certain delay when moved as will be explained below. Accordingly, when a chroma-key signal moves, an inconvenience may arise from the inability of the chroma-key signal to move in synchronism with the compressed picture.
FIG. 2A illustrates an output picture in a normal state and FIG. 2B, another output picture immediately after the movement of the chroma-key signal towards the right side of the frame. Since the compressed picture to be inserted into the chroma-key frame moves with a time lag, it is unable to change rapidly enough, resulting in an unnecessary picture position as is represented by hatching in FIG. 2B.
Moreover, since the brighter or white parts of the object in the picture involve much of the blue component the picture may include electrical, noise in addition to the desired key signal as illustrated in FIG. 2C. If the picture is compressed in accordance with such a chroma-key signal, it will be compressed into a picture which includes the noise component wherein the picture is the size represented by the dotted lines in FIG. 2C and appears like the picture in FIG. 2D. The insertion of the picture of FIG. 2D into the picture of FIG. 2C would result in a picture as shown in FIG. 2E, which is undersirable because of the failure of the inserted picture to be fully contained within the designated chroma-key frame. If the noise disappears, the picture will return to the state illustrated in FIG. 1E, but noise components are usually intermittent, resulting in alternating output pictures, which are very unpleasant to look at.
It is, therefore, an object of the present invention to provide a chroma-key tracking system which, when the chroma-key frame shifts in position, prevents unnecessary picture portions from being generated.
It is another object of the present invention to provide a chroma-key tracking system which is capable of correctly extracting the chroma-key frame when the video signal on which the chroma-key signal is based has a component which cannot be readily distinguished from the chroma-key signal.